Initiation protein Cdt1 was first identified in X. laevis, where it has been shown to be the second component of licensing factor (RLF-B) and in S. pombe. Cdt1 homologs have been identified in D. melanogaster, humans, and S. cerevisiae. Genetic studies in S. pombe have shown that binding of Cdc6 to chromatin requires the prior binding of Cdc18, the S. pombe homolog of Cdc6. In humans, the function of CDT1 is regulated during the cell cycle by its tight association with an inhibitory factor, geminin.
